Recently, for the purpose of reducing influences on the environment, the conversion of industrial processes to those in which various waste products are reduced as much as possible has been started, and interest in the removal of impurities has been growing. As a result, there is an increasing need for the development of a medium capable of adsorbing, concentrating, separating and removing the materials present in a trace amount in gases and liquids.
For example, hitherto, ion exchange resins have been mainly used for the removal of ions in the water in water treatment processes. Conventional ion exchange resins generally have a particle diameter ranging from 300-1200 μm. When such a resin is packed into a resin column and water to be treated is passed through the resin, the probability of contact of ions in the water to be treated with the surface of the resin is small because of large gaps among the resin particles (water flow path). Therefore, particularly the effect to remove ions from water with low ion concentration, such as ultrapure water, is conspicuously low. Furthermore, in the field of using ultrapure water, such as in the semiconductor industries, organic impurities (detected as TOC in the case of ultrapure water) incorporated within the resin during polymerization of the ion exchange resin elutes over a long period of time and thus impurities are brought into the treated water.
Attempts to use a sintered body as a medium for adsorption, removal and separation are disclosed in JP-A-51-23492, JP-A-7-204429, etc.
JP-A-51-23492 discloses a technique in which an adsorbent, such as silica gel or alumina, is mixed with a resin powder, such as a polyolefin resin, to prepare a slurry and this slurry is spread on a support and heated to obtain a sintered body. However, since the inorganic adsorbent, such as silica gel or alumina, could not be adhered to each other at the contacting portions, and, further, the adhesive force with thermoplastic resin powders was low, the adsorptive particles exfoliated from the sintered body. The sintered body thus could never be practically used.
JP-A-51-23492 and JP-A-7-204429 disclose a technique in which an ion exchange resin is mixed with thermoplastic resin particles and sintered, and the sintered mixture is used as a medium. However, since an ion exchange resin is used, the intrinsic problem of incorporation of impurities caused by elution of TOC and the like out of the resin has not yet been solved.
JP-A-57-109837 proposes a method of sulfonating a sintered porous body comprising a polyolefin resin with a sulfonating agent, such as fuming sulfuric acid, to make the sintered porous body hydrophilic. However, since the sintered resin per se is sulfonated, the exchange capacity is small. Besides, with a production method including sulfonation with, for example, fuming sulfuric acid or hot concentrated sulfuric acid, there is the problem of reverse contamination due to the elution of the impurities, i.e. the problem of the production of by-products produced in large amount by oxidation, dehydration, etc, has not been solved as in the case of using an ion exchange resin. In addition, there is a problem that fine particulate materials exfoliate from the resin that has become fragile by side reactions, such as oxidation and dehydration, and are brought into the treated water.
Therefore, in the field of water treatment, the ultrapure water production system using an ion adsorptive membrane has been studied and developed as a new deionization technique for meeting a need for producing ultrapure water which is less in elution of TOC and has high purity (JP-A-5-209071 and JP-A-7-41574). As compared with ion exchange resins, the ion adsorptive membranes have advantages of high ion removing efficiency and less elution of TOC, and can be used in the form of a flat film, a fiber, a hollow fiber, or the like. However, from the viewpoints of balances in separation function, water permeability and mechanical strength, the ranges of pore diameter and membrane thickness to be employed are restricted. In order to increase the surface area of a membrane, for example, when the membrane is in the form of a flat film, it had to be folded into a pleated form in a cartridge, and when it is in the form of hollow fibers, it had to be bundled and formed into a module. Although adsorptive bodies are primarily required to have high adsorption performance, since they are used for treating water or gases, they are not practical unless they are high in water permeability and gas permeability. In this respect a membrane with large thickness and large pore diameter is preferred. It is, however, difficult to produce such a resin by the above technique of producing ion adsorptive membranes.
The present invention provides an adsorptive structure that hardly generates impurities, such as TOC, does not contaminate the treated liquid or gas, and is capable of removing even impurities present at extremely low concentrations.